It is called the Chi-Rho, the first two Greek letters in
XPISTOS. It's usage goes back to very early Gentile Christianity,
perhaps Antioch. There were a number of symbols for Christianity
which Christians, during times of persecution, used to recognize
one another..perhaps painting them on the lintels of their doors as
a beacon for other traveling Christians. The grape cluster, anchor,
fish (IXQUS) and lamb being a few. The ChiRho was used extensively
in Constaninian times made famous by Constanine "bridge" vision.